Add bootstrap3 testcases

Split out code from lessc.test.testless into lessc.test.bootstrap for
reusability. The bootstrap3 test suite consists of two less files (that
@include all the others):

 - bootstrap.less
 - theme.less
Support function calls in expressions

LESS allows functions to be operators inside LESS operations and
bootstrap3 actually makes use of that.
Proper support for variables in escaped values.

The following constructs are escapes: ~"..." and ~'...'. Inside, only
@{...} is a LESS variable, @foo is not! To express that, the lexer needs
to additional states ('lessstringquotes' and 'lessstringapostrophe') and
some parsing rules special to those states. The literal '~' had to be
promoted to token too. Now, instead as marking the complete LESS string
as 'less_string' token, the lexer properly tokenizes it's content which
in turn the parser can properly evaluate.

Includes a minor bug fix, t_less_variable() wasn't ever used because
t_css_ident already captured them. This part was removed from
t_css_ident becaus it's much easier to overwrite t_less_variable in lexer
states.
LESS strings can contain $MS CSS filters

.klass {
  filter: ~'alpha(opacity=2)';
}
Only use css_ms_filter in fcall

The lexer catches Microsoft-specific function calls such "progid:..."
and "DX.Transform..." in the css_ms_filter token class. It should be
used only as the function for "fcall" constructs in the parser.
Support swapping "@{...}" type variables

These only occur in interpolated strings (~'...' or ~"...").
CSS strings can contain variables too

Ordinary CSS strings may contain @{...}-style variables. Here are some
examples:

    content: "@{breadcrumb-separator}\00a0";
    background-image: url("@{file-2x}");
    .col(@index + 1, ~"@{list}, @{item}");
Add t_tilde token for '~'

To be used inside general_sibling_selector (e.g. 'a ~ b { ... }')
instead of the literal '~'. Necessary since we had to remove the literal
from the lexer in order to tokenize ~"..." and ~'...'.